In my experience you won't get more accurate readings from a Magnetospeed or LabRadar than a ProChrono. I have tested more than one ProChrono by chronographing the same ammo (often at the same time) with a MS, LR and Oehler 35P, and all showed the same average velocity, given the differences in where they are from the muzzle.
There are other advantages to the more expensive models, especially if you're really into statistical analysis. But my ProChronos have provided average velocities good enough to get on small gongs easily at 500+ yards.
